1. Field of Invention
The invention concerns a transducer for bioacoustic signals comprising a transducer element having a front side and a rear side, the front side of which may establish an intimate contact with the surface of a body part, said surface being the transmitter of direct interior sound from the body, said transducer element being mounted in a housing subject to airborne noise, and having a surface surrounding the front side of said transducing element, said element and said surrounding surface being in intimate contact with the surface of said body part during use.
2. Description of Related Art
Transducers for bioacoustic signals emanating from a body usually belong to two main types applied to an outside surface of the body. One type is a microphone in principle, in which the vibration of a delimited area of skin is picked up as pressure variations in the air surrounding the area of skin, usually the pressure variations in a closed volume delimited by the skin, the microphone diaphragm, and the housing. An enclosed volume is essential to obtain a good low frequency response as well as protection from extraneous airborne noise—one early example is the standard binaural stethoscope in which the bell defines the volume. The second type is an accelerometer in principle, in which a light-weight housing rests against the part of the body and the inertial mass inside provides reference in the generation of signals proportional to the instant acceleration. This type has in itself a good protection against extraneous airborne noises, but the sensitivity decreases and the electrical noise increases very much in the lower end of the frequency range of interest, unless the inertial mass is increased to a value in which it unavoidably influences the actual measurement. There is hence a need for an improved transducer.
It has been determined that extraneous airborne noise in general enters the transducer mainly by two routes. One is direct airborne influence on the transducing element itself, e.g., a microphone diaphragm. The other is by means of influence on the diaphragm from the skin in touch with the diaphragm, while the housing is receiving airborne noise. A further contribution may be airborne noise radiated into the body around the housing surrounding the transducer in contact with the body, said airborne noise being converted to pressure waves which are re-radiated from the part of the skin directly in touch with the diaphragm. This type of noise injection is not avoided by enclosing the area of skin, however the phase relations to the desired signal are such that the contribution would generally be of minor importance. There is hence a further need to address in an improved construction of a transducer for bioacoustic signals.
In U.S. Pat. No. 5,610,987 a solution is given, which utilises a piezoelectric transflexural diaphragm in direct contact with the skin in the area within the surrounding housing. In this case, the noise signal is coupled to the diaphragm without re-radiation, and the rear of the diaphragm is shielded against extraneous noises by the housing. In order to obtain noise cancellation, this patent also describes that the housing contains an identical but outwards-facing piezoelectric transflexural diaphragm which is only subjected to airborne noise, and that a further identical transducer is placed in contact with the body some distance from the first transducer. Extensive digital signal processing enables a high degree of elimination of the undesired noises. This makes the equipment expensive and causes a need for re-programming if the sensor part is exchanged.
U.S. Pat. No. 6,028,942 relates to the chestpiece of an acoustic, non-amplified stethoscope having noise balancing means, in that a resonator is coupled to the reverse side of the diaphragm. An embodiment using amplification is also shown. The purpose is to compensate the noise that is radiated into the tissue surrounding the front end when applied to the skin and which is added to the desired signal. There is an air space between the tissue and the diaphragm that provides the output signal, however this remarkably reduces the usefulness of the device in practice. It is probable that the elaborate equivalent circuits used in U.S. Pat. No. 6,028,942 are misleading, because they do not take into account the pickup of airborne noise via the transducer housing mass itself.